Thermal transfer systems have been developed to obtain prints from pictures that have been generated electronically, for example, from a color video camera or digital camera. An electronic picture can be subjected to color separation by color filters. The respective color-separated images can be converted into electrical signals. These signals can be operated on to produce cyan, magenta, and yellow electrical signals. These signals can be transmitted to a thermal printer. To obtain a print, a black, cyan, magenta, or yellow donor layer, for example, can be placed face-to-face with a dye image-receiving layer of a receiver element to form a print assembly, which can be inserted between a thermal print head and a platen roller. A thermal print head can be used to apply heat from the back of the donor sheet. The thermal print head can be heated up sequentially in response to the black, cyan, magenta, or yellow signals. The process can be repeated as needed to print all colors, and a laminate or protective layer, as desired. A color hard copy corresponding to the original picture can be obtained. Further details of this process and an apparatus for carrying it out are contained in U.S. Pat. No. 4,621,271 to Brownstein.
Thermal transfer works by transmitting heat through the donor from the back-side to the donor layer. When the colorant in the donor layer is heated sufficiently, it sublimes or diffuses, transferring to the adjacent dye-receiving layer of the receiver element. The density of the colorant forming the image on the receiver can be affected by the amount of colorant transferred, which in turn is affected by the amount of colorant in the dye donor layer, the heat the dye donor layer attains, and the length of time for which the heat is maintained at any given spot on the donor layer.
At high printing speeds, considered to be 2.0 msec/line or less, the print head undergoes heat on/off cycles very rapidly. This generated heat must be driven through the donor support assemblage very rapidly to affect the colorant transfer from the donor to the receiver. Each layer in the donor can act as an insulator, slowing down the heat transfer through the layers of the donor to the receiver. Because of the short heat application time, any reduction in heat transfer efficiency results in a lower effective temperature in the donor layer during printing, which can result in a lower transferred colorant density. It is known to overcome the low print density associated with shorter line times by increasing the printhead voltage, increasing the relative amount of colorant in the donor layer, or a combination thereof. Applying higher print head voltages can decrease the lifetime of the thermal print head, and requires a higher power supply, both of which increase cost. Increasing the relative amount of colorant in the donor layer increases costs, as well as increasing the chance of unwanted colorant transfer, such as during storage of a donor element, as well as increasing donor-receiver sticking during printing.
To increase the thermal flux through the donor element, it is known to add thermally conductive agents, such as metallic particles, graphite, or carbon black, that will increase thermal conductivity of the support, intermediate layers, and dye donor layers. Increasing the thermal conductivity in the direction perpendicular to the plane of the donor element allows more heat to transfer through the thickness of the donor element in less time, enabling the use of less heat, less time, or both, to print an image. However, it is critical to minimize the amount of thermally conductive agents added to these layers to minimize any adverse effects that they may have on the physical and optical properties of the layers, and to keep costs low.
U.S. Patent Application Publications Nos. 2005-0134656A1 to Gao and 2005-0137089A1 to Gao et al. disclose that a polymeric material including thermally conductive inorganic particles can be in a layer between the donor layer and the support, the support, a layer beneath the support, or a combination thereof, to increase thermal conductivity of the donor element.
EP 0909659 discloses a dye donor element that has a support comprising a low thermal conductivity matrix and a plurality of high thermal conductivity particles dispersed in the matrix. This provides high thermal conductivity perpendicular to the plane of the donor element, and low thermal conductivity parallel to the plane of the donor element. There is a significant increase in materials cost as well as manufacturing challenges associated with this approach because the support is much thicker than any of the other layers in the donor element.
U.S. Pat. No. 6,476,842 B1 discloses the use of carbon black as a thermally conductive material in layers of a thermal donor other than the donor layer. US Patent Application Publication US2003/021153A1 discloses the use of carbon black or metals as thermally conductive patches in the donor layer. The addition of carbon black or colored metallic particles, such as Au, Ag, Cu, Pd, Pt, Ni, and graphite, can add color to the thermal donor element. This is undesirable because color sensors are used in the thermal dye transfer printers to detect which color patch is in line to be printed. Often, the donor element will contain a plurality of black marking lines that are utilized by the printer for alignment and registration purposes. If the dye donor layers become too black, such as when metallic or carbon black particles are added, then the printer cannot distinguish the colorant patches from the registration marks.
U.S. Pat. No. 4,826,717 discloses a dye layer containing inorganic particles of the same size as, or larger than, the thickness of the dye layer, wherein the entire dye layer is transferred to the dye receiving layer upon printing. There is a problem with this in that the particle size is too large. The particles provide thermal transfer pathways through the dye layer directly through the particles without actually heating the dye/binder part of the dye layer, which is necessary to effect dye transfer.
There is a need in the art for a means of increasing thermal conductivity of a material, such as for use in a donor element, increasing transferred colorant density of a donor element, and increasing print speed of a donor element while 1) maintaining or increasing print density, such as by increased colorant transfer efficiency, 2) maintaining or reducing power to the print head, 3) maintaining or reducing manufacturing cost, and 4) maintaining the optical properties (color and transparency) and physical properties of the donor colorant patches.